Update requests to use httr::RETRY and harmonise verbose messages

[stitam]

This PR addresses issues #257 and #268.

This PR updates most webservice requests to use httr::RETRY(). The main benefit is that it allows a webservice to be queried multiple times when the previous attempts were unsuccessful. This is especially useful when some webservices randomly become unavailable for a few seconds. Another benefit of the function is that it assigns HTTP status codes and status messages to all webservice requests, allowing harmonised verbose messages.

Feel free to add a comment/review/suggest alternatives.

PR task list:

• Update NEWS
• Add tests (if appropriate)
• Update documentation with devtools::document()
• Check package passed

[stitam]

I didn't touch ChemSpider, because I figured the functions can be simplified quite a bit. It's already done but I'll open a separate PR for that.

[stitam]

Travis is not working because Trusty is no longer supported, and I cannot get rJava to work under more recent versions. I'd rather let it go and fix Travis in a separate PR.

[andschar]

PR looks nice! Some minor comments.

[Aariq]

I love love love webchem_url(), and webchem_message(), and I love that now all of the functions use the same httr style including the HTML status reporting. Wonderful stuff. However, for multiple queries I still think most users would prefer that a function fail quickly if a service is down, rather than going very slowly and returning many NAs.

If you decide to keep this behavior, however, there are a number of places where the function still errors when a service is down (tested by turning off wifi and running example). I think it almost always happens because some tidying step or row-binding step is expecting something other than a length 1, unnamed vector. This could be fixed by returning a different data structure inside the if (inherits(res, "try-error")) statement. Alternatively, you could wrap foo() in try() or use purrr::possibly() rather than wrapping RETRY() in try() inside of foo(). So for example:

db_query <- function(query, verbose = TRUE) {

  foo <- function(query) {
    <function to do a single query.  Errors when service is down>
  }
  foo_safe <- possibly(foo, otherwise = tibble(query = NA, var1 = NA, var2 = NA), quiet = !verbose)
  out <- lapply(query, foo_safe)
}